Best Upgradeable DAC ?

I really like the idea of purchasing a DAC that is upgradeable which probably is making me lean towards Schitt Audio. Are there other mfrs that offer this service?
Depends on the upgrade model.

For those that require a factory rework, it might make sense if you lived near the manufacturer and it isn't expensive to ship to and from the manufacturer .

Others have dealer fitted upgrades like Bel Canto. I have a Bel Canto DAC3.5VB that I have upgraded from Mk I to Mk II specs and I recall Bel Canto offered upgrades for the DAC3 to DAC3VB and then a DAC3.x upgrade which fitted in 3.5 components. That was easy enough to do without a hefty shipping bill.

I think Wyred4Sound said their DAC2 was upgradeable too but I don't recall any upgrades being offered as yet.

If shipping is expensive (especially including insurance etc), most just trade the gear in for a new model :)
The best upgradeable DAC is so good it will never need to be upgraded. I don't know it'sd name but I am sure it is out there.
The Wyred4Sound dacs are upgradeable as they were designed that way.
Wyred4Sound claims they ICEBox MC power amps are also upgradeable but they need to be shipped back to them for upgrading.
Listening to the Schiit Gungnir as I write. Have had it for 24 hours now. It's very impressive. I'd give it a listen.
In principle the question of upgradable DACs seems to me as a bit of a conundrum. With the price of some very fine DACs in the $1K range, it would depend on the affordability of the upgrade plus the shipping involved. If an upgrade were offered which was $100-200 then perhaps even with shiping that route would be wise. However, it would be a very rare company which would offer such economical upgrades; more likely you'd be into it for approx. half the cost of the new unit by the time you were done.

However, if upgrades were in the $300-400 range the question should be asked, what is the holistic advantage of staying with the company upgrading when perhaps an entirely different DAC may offer far superior performance for about $500-600 more? With the sale of the owned DAC and avoidance of the upgrade fee the equation is not so skewed in favor of the upgrade as one may first think.

Frankly, I never buy gear based on upgradability. The market and technology is so fluid that while the one company is upgrading another has found a breakthrough technology which leaves the former in the dust.

Then, finally, even with upgrade, the unit may be easily bested by a competitor. Upgradability is no guarantee that one has the absolute best performance one can. It does assure that the march of technology does not completely pass you by, which is a good thing.
@douglas, your point makes sense. I totally agree with you, technology on this audio world keeps improving and you have to upgrade the whole machine not only certain parts to achieve the best performance.